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Archaeological Survey Technical Features Terms


Technical Features

NMR Magnetometer

The main sensors of the detection system are Nuclear Magnetic Resonance (NMR) Magnetometers. These highly sensitive sensors were developed by the French Atomic Energy Commission (CEA). Based on simultaneous proton and electron magnetic resonance (the Abragam-Overhauser effect), the NMR magnetometers measure more than one thousand times per second the absolute value of the earth's magnetic field - with an accuracy of one fifty-millionth of its value!

Theory

The probe is a field/frequency transducer based on the principle of nuclear magnetic resonance, amplified using dynamic electron polarization.

The hydrogen atoms of standard solvents have a magnetic moment which is proportional to their spin. In the earth's magnetic field, they precess around this field at a frequency proportional to the modulus - this is called the Larmor frequency, 1 to 3 Khz in the earth's field. The resonant electromagnetic excitation creates by spin phase coherence a macroscopic magnetization component precessing at the Larmor frequency. This component induces a voltage in the detection coil. The measured frequency gives the value of the field!

The nuclear magnetism is not directly detectable in the earth's field. The dynamic electronic polarization amplifies by a factor of a thousand the nuclear signal. The nuclear spins are coupled to the free electron spins of a radical in solution. Two excitation frequencies of the electronic resonance are possible: one gives a positive polarization, the other negative. The frequencies depend on the solvent used.

The correct choice of a pair of solvents, containing the same radical, will give - at the same frequency - a positive polarization factor in one and a negative one in the other (double effect).

The probe is composed of two hydrogenated solvents in individual flasks containing a free radical in solution, a high frequency dynamic polarization excitation circuit and a low frequency circuit, which simultaneously excites the nuclear resonance, and measures the signal. It is constructed of two symmetrical coils mounted in opposition.

MRM-2000 Frequency-meter The very accurate measurement and the conversion of the frequency signal issued from the probe is realized through a specialized frequency-meter allowing, with a sampling of 1000Hz (1 measurement every millisecond), the delivery of the absolute value of the magnetic field with an accuracy of one thousandth of a Gamma, which means an accuracy of a frequency measurement to the seventh decimal place (107 Hz).
A high-integrated version of this frequency-meter, named MRM-2000 has been developed by the engineering team of Franck Goodio in collaboration with the French Atomic Energy Commission.

Geomagnetism

The earth's magnetic field superposes very complex temporal and spatial phenomena - the earth's crust geology, dynamo effect, sun-earth interaction, ionospheric and telluric currents, etc. - based on magnetic and electric properties. The earth's magnetic field can be estimated as a bipolar field with a value between 20000 nano.Tesla (nT) on the equator and 60000 nT on the poles, added to a field of world-wide anomalies (of about 10000 nT), a field of geological local anomalies and temporary phenomena of a few tenths of nT per day.

Application to Archaeology

In archaeological remains, interesting localized magnetic anomalies, are superimposed on these natural anomalies. The discrimination between these anomalies is grounded in the high sensitivity of the sensors, and, when needed, a local magnetic gradient measurement between two simultaneously towed magnetometers. This gradiometer permits removal in real time of the temporal variations of the earth's magnetic field and permits us to reject a great proportion of the geological anomalies. This method, implemented with NMR magnetometers, enables the detection of very weakly magnetic objects, even those deeply buried in the sediments.



Sidescan Sonar

This seismic sensor provides an image of the acoustic contrasts on the sea-bed, on a band 50 metres on each side of the boat. Highlighting rocks or other prominent objects on the sea-bed, the sonar also gives an idea of their height by a measurement on the real time pulldown screen of the projected shadows. At the same time, the magnetometers will determine if a particular spot seen on the sonar is magnetic or not. The computer processing of the sonar data allows us to build a mosaic of the covered area by laying side by side the geographically positioned bands.



Echo Sounder

A precise bathymetric or relief map of the area is obtained by the implementation of echo sounders on the boat. These acoustic sensors deliver a non-stop accurate measurement of the depth along the survey lines.



Acoustic Positioning System

The system, called "short base acoustic positioning", is grounded on a regular time transmission of an acoustic signal from a transmitter - called a "pinger," and mounted on the mobile unit to be positioned - to an immersed boat-fixed receiver. Several pingers sending on request from the transceiver can be used at the same time. The positioning of each mobile unit is then calculated in terms of range and bearing seen from the receiver. Taking into account the geographical position of the boat recorded by the DGPS, and the known fixed position of the transceiver on the boat, the geographic position of all pinger-equipped sensors can be determined in real time.